Manufacturers of vehicles that employ internal combustion engines, more particularly diesel engines, are under increased pressure to comply with current and future emission standards for the release of oxides of nitrogen (NOx), particularly nitrogen monoxide (NO), as well as unburned and partially oxidized hydrocarbons (HC), carbon monoxide (CO), particulate matter, and other emissions, such as hydrogen sulfide (H2S) and ammonia (NH3). In order to reduce the previously mentioned emissions of a diesel engine, the diesel engine is typically operated with exhaust gas after-treatment system or systems through which the exhaust gas of the diesel engine flows and is treated.
Exhaust gas after-treatment systems typically include one or more after-treatment devices, such as oxidation catalysts, NOx abatement devices, diesel particulate filters (DPFs) and sulfur traps. These after-treatment devices generally require certain conditions to exist in the engine exhaust gas in order to perform optimally. More specifically, NOx abatement devices and oxidation catalysts, for example, have a relatively narrow temperature window within which the devices are activated, regenerated, or operate with high conversion efficiency. Periodically, after-treatment devices require heating of the exhaust gases and the device to temperatures that are higher than those typically provided by the engine in order to achieve the desired operating temperature of the after-treatment device. An example of one such device is a diesel particulate filter.
Modern compression ignition engines typically use exhaust after-treatment devices as a mechanism to satisfy stringent emission requirements. Some compression ignition engine exhaust after-treatment systems employ diesel particulate filters (DPF) or other devices, which elevate exhaust gas temperatures, in order to accomplish their function. In a DPF, this typically occurs during regeneration modes. There are two primary regeneration events for a DPF: passive and active. During passive regeneration, exhaust gases reach sufficient temperatures to promote catalytic reactions that oxidize trapped soot. In active regeneration modes, the onboard engine control module forces the engine to increase exhaust gas temperatures and/or regulate available oxygen content to either promote or halt a regeneration event. Regeneration events typically require exhaust temperatures between 570 and 650 degrees Celsius. When undergoing regeneration, exhaust gas temperatures must be managed, such as to avoid excessively high tailpipe gas temperatures. This becomes particularly important when the vehicle comes to a stop, while in regeneration mode. Most vehicle producers use a combination of software control strategies and mechanical devices to limit tailpipe exhaust gas temperatures.
Additionally, DPFs periodically require a relatively high concentration of oxygen in the exhaust gas to facilitate regeneration of the particulate filter. Often, the required exhaust gas conditions cannot always be achieved during normal operation of the engine. More particularly, the exhaust gas temperature can only be influenced to a certain degree by the combustion process without the use of a source of supplemental heat, such as an electric heater in the exhaust-gas stream. The particulate matter can generally be characterized as soot that is captured and reduced by a DPF. Present DPFs contain a separation medium with tiny pores that capture the soot particles. Resistance to exhaust flow in the DPF increases as trapped material accumulates in the DPF, thereby generating an increase in exhaust backpressure. The DPF must then be regenerated to burn off the particulate matter/soot in the particulate trap to reduce the exhaust backpressure and increase exhaust flow through the DPF. A typical method of regenerating a DPF utilizes an energy source such as a burner or electric heater to encourage combustion of the particulate matter. Particulate combustion in a DPF has been found to increase the temperature of the exhaust gas exiting the vehicle tailpipe downstream of the DPF.
Accordingly, it is desirable to provide a method and apparatus for cooling the higher temperature exhaust gas after it has exited the DPF or any other equivalent device without adversely affecting the engine performance.